GEOLOGY OF THE INNER EARTH—GREGORY. 819 
gists now justified in their belief that the deeper layers of the earth’s 
crust are in a state of fluxion, but, according to Arrhenius (1900), 
the earth is solid only to the depth of 25 miles, below which is a 
hquid zone extending to the depth of 190 miles; and below that level, 
he tells us, “ the temperature must, without doubt, exceed the critical 
temperature of all known substances, and at this depth the liquid 
magma passes gradually to a gaseous magma.” This distinguished 
physicist gives a description of the earth’s interior which reminds 
us of the views of the early geologists. Arrhenius’s theory rests, 
however, on the existence within the earth of exalted temperatures; 
and this assumption a geologist may now hesitate to accept with less 
risk of getting into disgrace than he would have run a few years ago. 
It is improbable that the rapid increase of heat with depth which is 
observed near the surface should continue below the lithosphere; for, 
if the earth consists in the main of iron, even although it be arranged 
as a mesh containing silicates in the interspaces, the heat conductivity 
might be sufficient to keep the whole metallic sphere at a nearly equal 
temperature. Here, again, Mr. Strutt’s work on radio-activity is in 
full agreement with the requirements of geologists, for he estimates 
that below a crust 45 miles thick the earth has a uniform temperature 
of only 1,500° C. Whether the further conclusion that this heat is 
due to the action of the radium in the crust be established or not, it 
is gratifying to hear a physicist arguing in favor of a moderate and 
uniform internal temperature. 
All that the actual observations prove and that geological theories 
require is that the material within the earth be intensely hot, and 
that it lie under such overwhelming pressure that it would as readily 
change its form and as quickly fill up an accessible cavity as any 
liquid would do. Whether such a condition is to be described as 
solid, liquid, or gaseous is of little concern to geologists. 
THE DEEP-SEATED CONTROL OVER THE EARTH’S SURFACE. 
The modern view of the structure of the earth adds greatly to the 
interest of its study, for it recognizes the world as an individual 
entity of which both the geological structure and the history have to 
be considered as a whole. Once the earth was regarded as a mere 
lifeless, inert mass which has been spun by the force of gravity, that 
hurls it on its course into the shape of a simple oblate spheroid. 
Corresponding with this astronomical teaching as to the shape of the 
world was the geological doctrine that all its topography is the work 
of local geographical agents, whose ‘control over the surface of the 
earth is as absolute as that of the sculptor’s chisel over a block of 
marble. 
